1. Field of the Invention
This invention relates to devices used for the sudden release of energy and in particular it relates to seismic sound sources such as air guns, used to release a pressurized gas such as air, into a body of water in order to observe the reflected or refracted waves resulting therefrom during seismic exploration.
2. Technical Description of the Prior Art
Conventional air guns contain a gas under pressure, usually air, compressed to 500 to 8000 lb/in.sup.2 (3.448.times.10.sup.6 to 55.16.times.106 Newtons/m.sup.2). A valve mechanism is mechanically, pneumatically, or hydraulically actuated to suddenly release the compressed air over a period of a few milliseconds to generate an acoustic impulse.
In general, a conventional air gun has at least two chambers: a control chamber and a firing chamber. Usually, the two chambers are in fluid communication with each other via restricted metering passageways so that the air pressure can be equalized between the two chambers. The firing chamber is provided with exhaust ports which, when the gun is cocked, are sealed by a valve such as a sliding sleeve, piston, or plunger valve. The valves, of whatever type, have two faces of different areas. The valve face exposed to the air pressure in the control chamber has the greater area. Accordingly, the differential forces acting against the two valve faces, hold the valve closed to seal the exhaust ports. To fire the gun, the differential forces are upset by lowering the pressure in the control chamber, or by slightly raising the pressure in the firing chamber. The pressure in the firing chamber forces the valve to abruptly open so that the air in the firing chamber explosively escapes through the exhaust ports. To open the exhaust ports, the valve moves in one direction only. Moving in the opposite direction, the valve closes the exhaust ports.
As noted above, valve members used in air guns may be of sleeve or plunger type and of the piston type. Sleeve or plunger valve members are moved by an actuator or driving mechanism against a seat to block the escape of high pressure air through the exhaust ports. The actuator then moves the valve member away from the seat in a direction parallel to the plane of the exhaust port. In the piston type, the valve member moves perpendicular to the plane of the exhaust port. In all types, the actuating or drive mechanism drives the valve member in one direction to unblock the exhaust ports. The actuator then reverses the direction of motion of the valve member to drive it back against the seat so that the firing chamber can be recharged. Valve actuators must control large forces and typically require several complex amplification stages to perform their task.
U.S. Pat. No. 3,638,752 to Wakefield shows an air gun using a cylindrical sliding sleeve valve that abuts against a seat to close the exhaust ports. U.S. Pat. No. 3,039,292 to Ottestod reveals a plunger-type valve member. Typical piston type valves are shown in U.S. Pat. No. 3,276,534 to Ewing and U.S. Pat. No. 3,310,128 to Chelminski.
The two major disadvantages of the conventional, above-described guns are related to the speed of operation and the complexity of the valve actuator or driving mechanism, required to control the large forces involved.
The prior-art air guns were wasteful of air because of their low speed of operation. The requirement for valve-member motion-reversal, that is, motion in a first direction to open the valve and motion in the reverse direction to close the valve, created an unavoidable lengthy period of time during which the exhaust ports remained open. Of course, compressed air bled from the firing chamber throughout the entire period that the valve was open during a firing cycle. It is known that it is only the initial outrush of air, which takes place just as the valve member first opens the exhaust ports, that generates useful acoustic energy. Further release of air contributes nothing to the strength of the acoustic pulse and is wasteful.
It is a general object of this invention to provide an air gun having faster valve operation so that it expends a minimum amount of compressed air during each firing cycle, thereby permitting use of a smaller air compressor than was hitherto deemed possible. Another object of this invention is to reduce the complexity of the gun and to eliminate metering passageways and multistage poppet and solenoid pilot valves.